1. Field of the Invention
The present invention relates to a projection type display apparatus capable of easily obtaining a large screen image, and more particularly to a constitution of projection type display apparatus and projection unit provided with a spatial light transmission system for transmitting and receiving video signals and audio signals by a spatial light transmission apparatus that does not require wiring.
2. Description of the Related Art
FIG. 1 is a schematic longitudinal sectional view showing a general constitution of a conventional projection type display apparatus. At the front side of a projection type display apparatus 1, a screen 5 is provided, and a mirror 4 is installed obliquely downward in its back side. Further at a lower side, a CRT 2 is provided at a specific angle to the mirror 4, and a projection lens 3 is disposed at the display surface side of the CRT 2.
The image formed on the CRT 2 is magnified by the projection lens 3, reflected by the mirror 4, and is magnified and projected on the screen 5. At this time, the video signal and audio signal are given from a signal source 6 such as VTR and optical disk connected by wire to the projection type display apparatus 1. Accordingly, the signal source 6 must be installed near the projection type display apparatus 1, and it was inconvenient to replace the tape or disk. Besides, since the viewer must replace the tape by hand, when the signal source 6 is located near the viewer, the signal wires may be laid around, which was not advantageous for the appearance of the room.
Recently, as the apparatus for solving this problem, a spatial light transmission apparatus is noticed. FIG. 2 is a structural drawing showing a conventional spatial light transmission apparatus. The spatial light transmission apparatus comprises a spatial light transmission transmitter (hereinafter called light transmitter) 7 connected to a signal source 6 disposed apart from a projection type display apparatus 1, and a spatial light transmission receiver (hereinafter called light receiver) 8 disposed on the top of the projection type display apparatus 1, being connected by using a short signal wire. The light transmitter 7 modulates the video signal and audio signal by a modulation circuit, and converts into light signal by a light source 9 such as light emitting diode, and emits a luminous flux 10. FIG. 3 is a schematic diagram showing the constitution of the light receiver 8. The light receiver 8 comprises a photoreceptor 21 including a photo detector 11, and a demodulation circuit. In the light receiver 8, the photo detector 11 receives the luminous flux 10, converts photoelectrically, then demodulates the signal in the demodulation circuit, and gives to the projection type display apparatus 1.
In such spatial light transmission apparatus, it is not necessary to install a signal source around the signal source 6 and projection type display apparatus 1, and they can be separated in space. Therefore, when viewing the projection type display apparatus 1 from a relatively distant place, it is not necessary to lay down a long signal wire, and it is particularly advantageous.
FIG. 4 is a diagram showing the state of appreciating the picture by the projection type display apparatus. The projection type display apparatus 1 can show a large screen of about 40 inches to 70 inches on a screen 5 by its projection unit, and it is a display apparatus suited to be viewed by a multiplicity of audience at a position several meters apart, but when occupied by one person or to prevent the sound from leaking outside, as shown in FIG. 4, it is often enjoyed by connecting a headphone so that the sound may not be heard by the others. In such a case, the viewer puts on a headphone 31 connected to the projection type display apparatus 1 by way of a signal wire 32.
However, when the viewer appreciates from a position several meters apart from the display type projection apparatus 1, it is necessary to connect a long signal wire 32 of several meters from the projection type display apparatus 1. Such case was accompanied by the trouble of wire laying, deterioration of signal during transmission through the signal wire 32, and restriction of the motion of the viewer by the length of the signal wire 32, among others.
The spatial light transmission apparatus is noticed as an apparatus for solving such problems. FIG. 5 is a diagram showing other constitution of such conventional spatial light transmission apparatus. This space-transmission apparatus comprises a spatial light transmission transmitter (hereinafter called light transmitter) 33 connected to the top of the projection type display apparatus 1 through a short signal wire, and a spatial light transmission receiver (hereinafter called light receiver) 34 connected to the headphone 31. The light transmitter 33 modulates the audio signal by the modulation circuit, converts into light signal by a light source such as light emitting diode, and emits a luminous flux 35. In the light receiver 34, the photo detector receives the luminous flux 35, converts photoelectrically, demodulates the signal in the demodulation circuit, and transmits a desired audio signal to the headphone 31. Since the spatial light transmission apparatus can transmit audio signal to the viewer without resort to wire, long wiring installation is not needed.
In the conventional projection type display apparatus 1 as shown in FIG. 2, however, the luminous flux utility rate is low due to the spread of the luminous flux 10 of the light transmitter 7 and the smallness of the light receiving area of the light receiver 8, and this caused a problem that the spatial light transmission efficiency in a long distance was poor. Besides, the light receiver 8 or light transmitter 33 is disposed on the projection type display apparatus 1, which was not favorable For the appearance.
A main cause of low utility rate of luminous flux is that the light emitting diode 9 as light source generally includes a half-power spreading angle of 5 degrees to 10 degrees or more, that is, in the case of spatial light transmission for about 5 meters, the diameter of the luminous flux is about 2 meters. On the other hand, the photo detector 11 is required to have a fast response to the video signal, and hence its area is limited, and specifically it is defined within about. 3 mm to 5 mm. Therefore, since the utility efficiency of luminous flux is the area ratio, only about 10E to 6 can be utilized. To compensate for such low luminous flux utility efficiency, as conventional examples of improvement, plural light emitting diodes 9 were used the transmitter 7, or a condensing lens 12 with an aperture of about 10 mm to 20 mm is disposed at the front side of the photo detector 11 of the photoreceptor 21 as shown in FIG. 3, thereby increasing the apparent light receiving area.
At this time, however the light transmitter and receiver 7, 8 become large in size, and the cost is increased, and the incident angle of the luminous flux 10 that can be received is limited, and these problems remain unsolved. That is, as shown in FIG. 3, the luminous flux with an incident angle exceeding angle .theta. determined by the focal length of the condensing lens 12 and size of the photo detector 11 cannot be received by the photo detector 11. Therefore, to confront the direction of the exit luminous flux 10 from the light transmitter 7 in FIG. 2, there arised such problems that it was necessary to adjust the direction of the photoreceptor 21.
Moreover, in the conventional projection type display apparatus 1 shown in FIG. 5, the position of the audience was limited by the directivity of the luminous flux 35. Yet, same as in FIG. 2, the light receiver 8 or light transmitter 33 is disposed on the projection type display apparatus 1, which was not favorable for the appearance.
Generally, the light emitting diode of the light source used in the transmitter 7 includes a half-power spreading angle of 5 degrees to 10 degrees or more, and in the case of spatial light transmission for about 5 meters, for example, the diameter of the luminous Flux is about 2 meters. On the other hand, the projection type display apparatus 1 includes a wide viewing range of about 140 degrees in the horizontal direction, and about 30 degrees in vertical direction, and hence there arised such problems that it was necessary to adjust the direction of the light transmitter 33 so that the optical axis 36 which is the center line of the luminous flux 35 may be directed to the light receiver 34, in order to receive the luminous flux 35 depending on the position of the viewer.
A constitution of a conventional three-tube projection type display apparatus having three monochromatic CRTs as image forming means is explained by reference to FIG. 6. Reference numerals 71R, 71G, and 71B are CRTs (cathode ray tubes) comprising image forming means for red, green and blue, respectively, and projection lenses 72R, 72G, and 72B are disposed at the face plate side of the CRTs 71R, 71G, and 71B through spacers 74R, 74G, and 74B, respectively. The optical axes 75R, 75G, and 75B of the lights emitted from the CRTs 71R, 71G, and 71B and passing through the spacers 74R, 74G, and 74B and projection lenses 72R, 72G, and 72B are projected on a screen 73 at specific angles.
CRTs are monochromatic CRTs 71R, 71G, and 71B corresponding to the three primary colors of red, green, and blue, and, as shown in FIG. 6, images drawn on CRTs 71R, 71G, and 71B are magnified and projected by the projection lens 72 and formed on the screen 73. At this time, the projection unit including the CRT 71 and projection lens 72 is disposed so that the optical axes 75R, 75G, and 75B may intersect nearly in the middle of the screen 73, and it is constituted so that the projection distances from the projection lenses 72R, 72G, and 72B along the optical axes 75R, 75G, and 75B to the screen 73 may be substantially equal.
The reasons why the focus adjustment is necessary are explained below. FIG. 7 is a longitudinal sectional view of the projection unit shown in FIG. 6. The projection lens 72 is composed of lens elements L1 to L6 disposed sequentially from the screen side, being supported by a lens barrel 78. The lens barrel 78 supports the lens elements L1 to L4 and lens elements L5, L6 separately, and incorporates a focal adjusting mechanism 79 for adjusting the interval between the lens element L4 and lens element L5 by a screw. The face plate 76 of the CRT 71 and the lens barrel 78 of the projection lens 72 are fixed to the spacer 74. The space enclosed by the face plate 76, the lens element L6 located at the position closest to the CRT side, and the spacer 74 is filled with a coolant 77.
According to the first reason why the focal adjustment is necessary, the projection lens 72 is designed at the specific projection distance and magnification, and actually when setting up the projection type display apparatus, an error of several hundred microns is caused, and when forming the image of the CRT 71 on the screen 73, a delicate focus deviation is caused, and the design performance cannot be achieved. Generally, therefore, a focal adjusting mechanism 79 as shown in FIG. 7 is provided, and the focal deviation is corrected by inching the lens barrel 78 portion which supports the lens elements L1 to L4, out of the plural lens elements L1 to L6 which compose the projection lens 72.
In the second reason why the focal adjustment is necessary, chromatic aberration is not corrected generally in the projection type display apparatus for consumer use for the purpose of cost reduction, and the projection lens 72 is varied in the focal length by the wavelength of the passing luminous flux, that is, the color light. In this prior art, since projection lenses of same composition are used in optical system of red, green, and blue colors, for example, the projection lens designed at the wavelength of green color is deviated in focus in the projection unit of red and blue. Hence, using the focal adjusting mechanism 79, it has been adjusted to obtain the best image-forming performance.
Of the plural lens elements L1 to L6 for composing the projection lens 72 for adjusting the focus, only part of them (L1 to L4) are moved, which is because coolant 77 is sealed between the projection lens 72 and face plate 76, and since the final lens element L6 is in contact, at least the final lens element L6 cannot be moved for the purpose of focal adjustment.
The conventional CRT projection type display apparatus had such constitution and action, and hence the following problems existed.
Since the projection lens 72 is in contact with the coolant 77, it is necessary to move part of the lens elements for focal adjustment. The focal adjustment for absorption of the disposition error as the first reason for requiring adjustment corresponds at most to several hundred microns or several millimeters at maximum on the screen, and the focal deviation on the screen due to difference in the wavelength or color light as the second reason for requiring adjustment was scores of millimeters to 10 cm in the projection type display apparatus of the screen size of about 40 inches to 50 inches, and this large focal deviation about 100 times of the disposition error must be absorbed by the adjustment mechanism of projection lens. As a result, in the constitution of the projection lens in the projection unit of red, green and blue, in FIG. 7, three colors could not be image-formed correctly unless the interval of the lens elements L4 and L5 was largely different.
In the projection lens, however, it is generally designed at a specific color, for example, the wavelength of green, and the shape and interval of lens elements are determined so as to achieve the optimum aberration correction at specific projection distance and magnification. Hence, in the red or blue projection unit in FIG. 7, when the interval of L4 and L5 is moved largely, the optimum aberration correction adjusted to the optimum position by focal adjustment is deviated, and the design performance is not obtained in other projection unit than green.
For example, the resolution characteristic (MTF) of the projection lens in FIG. 7 designed in green (545 nm) is shown in FIG. 8, and the resolution characteristic at the best focal position when using the same projection lens in red (610 nm) is shown in FIG. 9. In the diagrams, the axis of abscissas denotes the image height from the image center to the most peripheral area, the broken line indicates the MTF in the meridional direction (Mer), and the solid line, in the sagittal direction (Sag). When used in the red projection unit, a large performance deterioration may occur in the meridional direction in the image peripheral area, in particular.
The cause of this phenomenon lies in the reduction of size and heightening of brightness of the recent projection type display apparatus, and there is a strong demand for wider angle of view and lower F value in the projection lens. Accordingly, as shown in the example in FIG. 7, aspherical lenses are used in the lens elements L1, L2, L5, L6, to correct the aberration powerfully. In addition, the aspherical area is wide, and even a visible inflection point is formed in the peripheral pat of the aperture, in particular. Hence, the aberration correction is likely to be broken by a slight change in the relative position of the lens elements.
FIG. 10 is a schematic diagram showing an optical path in the projection lens 2, indicating only the parts of lens elements L4, L5. On a fluorescent plane 80 of the CRT 1, the luminous flux 12 emitted from the emission point 81 in the peripheral part of the screen is refracted by the lens elements L5, L4. Numeral 83 denotes the position of the lens element L4 in the design state designed for the green projection unit, and 84 indicates the position of the lens element L4 in the red projection unit. In the red projection unit, since the focal length is long, the lens elements L1 to L4 must be set remote from the lens element L5 for focal adjustment. The luminous flux 82 entering obliquely from the emission point 81 in the peripheral area of the screen differs in the incident position and angle from the design state along with the parallel move of the lens element L4. Hence, there arises a problem that the aberration correction in the design state is broken, and the picture quality deteriorates especially in the peripheral area of the screen.
This is a common problem in the projection unit having projection lens using aspherical lens, generally, not limited to the lens type shown in the prior art. Of course, the problem can be solved by designing the lenses separately for red, green and blue, or by using the lenses in consideration of the chromatic aberration correction, but it is too costly for the projection type display apparatus for consumer use, and it is an unrealistic measure.